By combining data for more than 250,000 individual objects, an MPA-based team has for the first time been able to measure X-ray emission in a uniform manner for objects with masses ranging from that of the Milky Way up to that of rich galaxy clusters. The results are surprisingly simple and give insight into how ordinary matter is distributed in today's universe, and how this distribution has been affected by energy input from galactic nuclei.
While galaxies, with their billions of stars, may seem unfathomably
large, the Universe contains even bigger objects. Clusters of galaxies
are the largest known equilibrium structures. They can contain many
hundreds of galaxies and a total mass thousands of times that of the
Milky Way system. Galaxies and clusters appear very different when
viewed in optical light (see Fig. 0), but computer simulations such as
the Millennium Simulation suggest that their dark matter distributions
should look very similar. The technical term for this is
'self-similarity', which in this context means that the dark matter
halos of galaxy clusters are more or less just scaled-up versions of
those which surround galaxies.
Both galaxies and galaxy clusters (and their dark matter halos) are
expected to be suffused with hot gas as well. This gas, which is heated
to temperatures of millions of Kelvin, emits high-energy radiation and
can be studied with X-ray telescopes like ROSAT and XMM-Newton. Studies
of dozens of galaxy clusters show that the X-ray luminosity of the hot
gas increases with the total mass of the cluster. Independently, studies
of dozens of elliptical galaxies have shown that the X-ray luminosity
of their hot gas increases with the stellar mass of the galaxy. These
two correlations connect X-ray luminosity to two different quantities
(total mass for clusters, stellar mass for galaxies), and have typically
been measured in different ways for the different types of object.
A team at MPA has now combined these two relations using an archived
X-ray map of the whole sky. They analysed emission around a sample of
250,000 galaxies in the ROSAT All-Sky Survey - more than a thousand
times the number used in any previous galaxy study - and carefully
combined the X-ray emission from several thousand similar mass galaxies
into a set of average images in a process known as "stacking". Example
stacked images are shown in Figure 1 for two different stellar masses.
By eye, the distribution of the hot gas in galaxy clusters looks just
like a scaled-up version of that around much smaller galaxies. The full
results are shown in Figure 2, which shows the relation between mean
X-ray luminosity and stellar mass. This relation follows a straight line
all the way from the individual galaxy regime (small masses) up to the
rich cluster regime.
However, a more detailed analysis shows that the slope of this line is
steeper than would be expected if the hot gas were perfectly
self-similar. This is probably due to a combination of effects, with a
major contribution coming from heating by supermassive black holes at
the centres of galaxies. As gas falls into a supermassive black hole, it
loses large amounts of energy which are pumped into the hot gas
atmosphere surrounding the galaxy. This is known "active galactic
nucleus (AGN) feedback" and is thought to be important in the formation
of both galaxies and galaxy clusters. AGN feedback has a bigger effect
on less massive systems, lowering the X-ray luminosity of galaxies much
more than that of clusters.
This effect makes the relation in Figure 2 steeper than it would be if
the hot gas were perfectly self-similar. The new measurements of X-ray
luminosity over a broad range of masses gives a powerful clue to help
understand AGN feedback. Comparing these measurements against
predictions from numerical simulations, the MPA team showed that gentle,
'self-regulated' AGN feedback is preferred over more violent input of
energy.
Detailed comparison with previous measurements show that the new results
are perfectly consistent with previously measured scaling relations for
galaxies, as well as with scaling relations measured for optically
selected samples of galaxy clusters. This suggests that a single
relation can indeed describe both types of object. Studies of scaling
relations for galaxy clusters selected by their X-ray properties have
typically shown a similar slope but a systematically higher mean
brightness at given total mass. This is most likely a reflection of the
diversity of X-ray properties among clusters of a given total mass,
which may have been underestimated in earlier work.
Finally, this work complements a similar analysis performed for the same
galaxies and galaxy clusters using data from the Planck satellite. That
analysis used the shadows which hot gas atmospheres cast on the cosmic
microwave background to measure the total thermal energy of the hot gas,
as opposed to its X-ray luminosity, finding this to scale with mass
self-similarly. Combining these two results implies that a large
reservoir of hot gas surrounds galaxies, but is too rarefied as a result
of AGN feedback to emit strongly in X-rays. This would resolve the
long-standing problem of the location of the baryons which "should" be
associated with the galaxies but had not previously been detected
directly.
Mike Anderson, Massimo Gaspari, Simon White (MPA), Wenting Wang (Institute for Computational Cosmology, University of Durham), Xinyu Dai (Department of Physics and Astronomy, University of Oklahoma)
Mike Anderson, Massimo Gaspari, Simon White (MPA), Wenting Wang (Institute for Computational Cosmology, University of Durham), Xinyu Dai (Department of Physics and Astronomy, University of Oklahoma)
Publications:
Anderson, Michael E.; Gaspari, Massimo; White, Simon D. M.; Wang, Wenting; Dai, Xinyu Unifying X-ray scaling relations from galaxies to clusters,
MNRAS Volume 449, Issue 4, p.3806-3826 (2015)
Planck Collaboration Planck intermediate results. XI. The gas content
of dark matter halos: the Sunyaev-Zeldovich-stellar mass relation for
locally brightest galaxies,
Astronomy & Astrophysics, Volume 557, id.A52, 17 pp. (2013)
